Traction control system including converter protection function

ABSTRACT

A method and a device for protecting a torque converter of an automatic transmission from overheating during standing-start of a motor vehicle, a slipping wheel being decelerated by a traction control system by a braking intervention. To improve the protective function of the traction control system, the energy loss converted in the torque converter or a value proportional to it is calculated and the engine torque is reduced if a specified threshold value is exceeded.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for protecting a torqueconverter from overheating and a traction control system including aconverter protection function.

BACKGROUND INFORMATION

[0002] When starting off on a road surface having different adhesivefriction values between the right and left sides of the vehicle(μ-split), even when a relatively slight drive torque is applied, thewheel (low-μ wheel) located on the slippery side of the road surfacebegins to spin. If the speed of the low-μ wheel exceeds a specific slipthreshold, the traction control system (TCS) intervenes in the operationof the vehicle and brakes the slipping wheel.

[0003] The braking torque exerted on the low-μ wheel is transferred viathe differential to the wheel on the non-skid side of the road surface(high-μ wheel) and may be used there for the propulsion of the vehicle.

[0004] With a regulation of this type, the brake intervention on theslipping wheel converts the engine torque in the brake into heat. Invehicles with automatic transmission, this also has the consequence thatthe engine torque produced (less the torque acting in the brake) isconverted into thermal energy in the torque converter of the automatictransmission, which may destroy the torque converter or the transmissioneven after a relatively short time.

[0005] During standing-start operations under μ-split conditions,situations may arise in particular with heavily loaded vehicles orvehicles with trailers in which the vehicle does not begin to movedespite maximum drive torque, since the braking resistance torque androlling resistance torque of the vehicle and the downgrade force actingon the vehicle and trailer are greater than the engine torque producedby the engine. In such situations, the result is an extreme load and acorrespondingly rapid overheating of the torque converter.

[0006] Other traction control systems therefore include a converter ortransmission protection function, which is implemented using a timecounter, which is started in the control state “select high” whenstarting off on μ-split and below a vehicle speed of 5 km/h and whichcauses a forced switch into the state “select low” after a specifiedperiod of time (e.g., 15 seconds).

[0007] The state “select high” of a TCS is used to attain the maximumpossible traction and is characterized by high slip thresholds for thedrive wheels and a relatively high delivery of engine torque. Incontrast, the state “select low” is used to attain the maximum possiblevehicle stability and is characterized by slip thresholds set to be verysensitive and a correspondingly low engine torque.

[0008] The protective function of other systems provide that the TCSperforms a rigid (time-controlled) switch independent of the actual loadon the torque converter. Thus the speed of a vehicle moving slowlyforward with slipping drive wheels may be limited just before reaching anon-skid road surface, although the temperature reached in the torqueconverter would not have required this yet.

SUMMARY OF THE INVENTION

[0009] The present invention provides a converter protection functionfor a traction control system to the effect that the engine torque isreduced when it is actually necessary to protect the converter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 shows a flow chart to explain a method for protecting atorque converter from overheating according to one exemplary embodimentof the present invention.

[0011]FIG. 2 shows a schematic representation of a traction controlsystem.

DETAILED DESCRIPTION

[0012] The sequence of a method for protecting a torque converter fromoverheating is shown in FIG. 1 in the form of a flow chart. In a firststep 1, the energy loss M_(CONVERTER) _(—) _(LOSS) or a valueproportional to it is calculated and this value is compared with aspecified threshold value in Step 2.

[0013] If the energy loss converted in the torque converter is greaterthan threshold value sw, the engine torque is reduced in step 3,otherwise the current engine torque is maintained.

[0014]FIG. 2 shows a traction control system 4 including a device 5 fordetermining the energy loss converted in the torque converter or a valueproportional to it. The current braking torque M_(BRAKE) and the currentengine torque M_(ENGINE) are supplied to traction control system 4. Aswas described above, engine torque M_(ENGINE) is reduced if theconverter energy loss or the proportional value exceeds a specifiedthreshold value.

[0015] In particular, an exemplary embodiment includes calculating theenergy loss converted in the torque converter or a value proportional toit and reduce the engine torque if the energy loss or the proportionalvalue exceeds a specified threshold value. The energy loss or theproportional value represents a measure of the temperature prevailing inthe torque converter, which is accordingly only reduced if the convertertemperature requires it.

[0016] Also, according to an exemplary embodiment, a torque balance isperformed relative to the propulsion and deceleration moments acting onthe vehicle, from which a converter torque loss is determined. Theconverter torque loss is integrated over time and thus forms a referencevalue, which is a measure of the energy dissipation converted in thetorque converter and accordingly the converter temperature.

[0017] In calculating the energy loss converted in the torque converteror the proportional value, the heat dissipation of the torque convertermay be taken into consideration.

[0018] The TCS, which is in the state “select high” at the beginning ofthe starting-off operation, may switch to the state “select low” if theenergy loss converted in the torque converter or the value proportionalto it exceeds the specified threshold value.

[0019] During standing-start under μ-split conditions, a wheel locatedon the slippery side of the road surface begins to slip due to theengine torque applied by the driver. The TCS recognizes this andregulates an appropriate brake pressure on the low-μ wheel. Since thevehicle does not drive off, the engine torque delivered is convertedinto heat in the torque converter and in the brake and into accelerationin the drive train. The following relationship applies:

M _(ENGINE) =M _(BRAKE) +M _(CONVERTER) _(—) _(LOSS) +M _(WBR), where

[0020] M_(ENGINE): engine torque produced

[0021] M_(BRAKE): braking torque converted in the brake

[0022] M_(CONVERTER) _(—) _(LOSS): torque loss which heats the converter

[0023] M_(WBR): rotational acceleration resistance torque of the drivetrain

[0024] The acceleration of the drive train lasts for only a short periodof time. Afterwards, a steady state prevails, i.e., the angularacceleration is equal to zero as is the rotational accelerationresistance torque M_(WBR) as well. In the steady state, the followingequation applies to the torque loss of the converter M_(CONVERTER) _(—)_(LOSS):

M _(CONVERTER) _(—) _(LOSS) =M _(ENGINE) −M _(BRAKE)

[0025] For the brake pressure delivered by the TCS, the followingapplies:

M _(BRAKE) =P _(BRAKE) *C

[0026] C being a conversion constant (Nm/bar).

[0027] Taking into consideration the rotational acceleration resistancetorque M_(WBR) of the drive train, the following applies:

M _(WBR) =M _(WBR) _(—) _(ENGINE) +M _(WBR) _(—) _(DRIVETRAIN) with

M _(WBR) =J _(ENGINE)*ω_(ENGINE) +J _(DRIVETRAIN)*ω_(DRIVETRAIN), where

[0028] J: mass moment of inertia [kgm²]

[0029] ω: angular acceleration [l/s²]

[0030] Both the engine speed and the wheel speeds are known as are theindividual mass moments of inertia J of the drive train.

[0031] To determine the energy loss converted in the converter, theconverter torque loss M_(CONVERTER) _(—) _(LOSS) is integrated overtime. In doing so, the energy dissipation, through convection andthermal radiation in particular, which causes a reduction intemperature, may also be considered. With iterative calculation (indext), the following applies:

M _(CONVERTER) _(—) _(LIMIT) =M _(CONVERTER) _(—) _(LIMIT(t−1)) +dM_(CONVERTER) _(—) _(LOSS) *dt−dM _(CONVECTION) *dt, where

[0032] dMCONVEcTION: application parameter.

[0033] If converter limit torque M_(CONVERTER) _(—) _(LIMIT), which isspecified as the threshold value, is exceeded the operating state of theTCS is switched from “select high” to “select low,” i.e., the engineregulates very sensitively.

[0034] According to another exemplary embodiment, a temperature modelmay be used to determine the temperature of the torque converter. Thetemperature model is a computer model which describes the thermalcharacteristics of the torque converter under different propulsion andbraking conditions. As an input variable, the temperature modelincludes, for example, the converter torque loss and may also take theheat dissipation into consideration.

[0035] The TCS may switch from “select high” to “select low” if thetemperature determined by the temperature model exceeds a specifiedthreshold value.

What is claimed is:
 1. A method for protecting a torque converter of anautomatic transmission of a vehicle from overheating during a tractioncontrol operation, the method comprising: calculating one of an energyloss converted in the torque converter and a value proportional to theenergy loss converted in the torque converter; and reducing an enginetorque when one of the energy loss converted in the torque converter andthe value proportional to the energy loss converted in the torqueconverter exceeds a threshold value.
 2. The method of claim 1, furthercomprising: performing an engine torque balance relative to a propulsionand deceleration moments acting on the vehicle, and determining aconverter torque loss therefrom.
 3. The method of claim 2, wherein theconverter torque loss is integrated over time.
 4. The method of claim 1,wherein, in calculating the one of the energy loss converted in thetorque converter and the value proportional to the energy lossconverted, a heat dissipation of the torque converter is considered. 5.The method of claim 1, wherein the traction control system switches froma select high state to a select low state when the one of the energyloss converted in the torque converter and the value proportional to theenergy loss converted in the torque converter exceeds the thresholdvalue.
 6. A traction control system for protecting a torque converter ofan automatic transmission from overheating during a traction controloperation, comprising: a device to determine one of an energy lossconverted in the torque converter and a value proportional to the energyloss converted in the torque converter; and an arrangement to reduce anengine torque when the energy loss converted in the torque converter andthe value proportional to the energy loss converted in the torqueconverted exceeds a threshold value.